1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to roller cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to a cutter element having a bowed crest geometry which provides for a more uniform stress distribution.
2. Background Art
The success of rotary drilling enabled the discovery of deep oil and gas reserves. The roller cone rock bit was an important invention that made that success possible. The original roller-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, was able to drill the hard caprock at the Spindletop field, near Beaumont, Tex.
That invention, within the first decade of the twentieth century, could drill a scant fraction of the depth and speed of modern rotary rock bits. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill overall improvement in the performance of rock bits.
Roller-cone rock bits typically are secured to a drill string, which is rotated from the surface. Drilling fluid or mud is pumped down the hollow drill string and out of the bit. The drilling mud cools and lubricates the bit as it rotates and carries cuttings generated by the bit to the surface.
Roller-cone rock bits generally have at least one, and typically three roller cones rotatably mounted to a bearing on the bit body. The roller cones have cutters or cutting elements on them to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during drilling operation. These stresses cause the rock to fail, resulting in disintegration and penetration of the formation material being drilled.
Operating in the harsh down hole environment, the components of roller-cone rock bits are subjected to many forms of wear. Among the most common forms of wear is abrasive wear caused by contact with abrasive rock formation materials. Moreover, the drilling mud, laden with rock chips or cuttings, is a very effective abrasive slurry.
Many wear-resistant treatments are applied to the various components of the roller-cone rock bit. Among the most prevalent is the application of a welded-on wear-resistant material or “hardfacing.” This material can be applied to many surfaces of the rock bit, including the cutting elements.
U.S. Pat. No. 4,262,761 discloses a milled steel tooth rotary rock bit wherein one or more holes are drilled into the crest of the tooth-shaped cutting structure. Tungsten carbide rods are positioned in the holes and hardfacing is applied to the tooth. The hardfacing is applied across the top of the tooth crest and acts to hold the tungsten carbide rods in place. The rods are inserted in holes parallel and close to one flank of the tooth so that the entire length of the carbide rods can be attached to the hardfacing by burning the hardfacing through to the carbide rods. Wear on the tooth will proceed along the side of the tooth not reinforced with the carbide rods and a self-sharpening effect is enhanced by the strength of the carbide rods. The carbide rods and holes therefore can be relatively inexpensive, since close tolerance finishing is not required.
U.S. Pat. No. 5,152,194 discloses a milled tooth roller cone rock bit consisting of chisel crested milled teeth with generously radiused corners at the ends of the crest. A concave depression is formed in the crest between the radiused ends. A layer of hardfacing material formed over each tooth is thicker at the corners and in the concave depressions in the crest to provide a means to inhibit wear of the hardfacing as the bit works in a borehole.
U.S. Pat. No. 5,311,958 discloses an earth-boring bit that is provided with three cutters, two of the three cutters are provided with heel disk cutting elements defined by a pair of generally oppositely facing disk surfaces that generally continuously converge to define a circumferential heel disk crest. One of the two cutters having heel disk elements is further provided with an inner disk cutting element.
U.S. Pat. No. 5,492,186 discloses an earth boring bit rotatable cutter having a first hardfacing composition of carbide particles selected from the class of cast and macrocrystalline tungsten carbide dispersed in a steel matrix deposited on the gage surface of at least some of the heel row teeth. A substantial portion of these particles are characterized by a high level of abrasion resistance and a lower level of fracture resistance. A second hardfacing composition of carbide particles selected from the class of spherical sintered and spherical cast tungsten is dispersed in a steel matrix deposited over at least the crest and an upper portion of the gage surface to cover the corner that tends to round during drilling. A substantial portion of the particles of this composition are characterized by a high level of fracture resistance and a lower level of abrasion resistance.
U.S. Pat. No. 5,868,213 discloses a steel tooth, particularly suited for use in a rolling cone bit, includes a root region, a cutting tip spaced from the root region and a gage facing surface therebetween. The gage facing surface includes a knee, and is configured such that the cutting tip is maintained at a position off the gage curve. So positioned, the cutting tip is freed from having to perform any substantial cutting duty in the corner on the borehole corner, and instead may be configured and optimized for bottom hole cutting duty. The knee on the gage facing surface is configured and positioned so as to serve primarily to cut the borehole wall. It is preferred that the knee be positioned off gage, but that it be closer to the gage curve than the cutting tip.
U.S. Pat. No. 6,206,115 discloses an earth-boring bit having a bit body with at least one earth disintegrating cutter mounted on it. The cutter is generally conically shaped and rotatably secured to the body. The cutter has a plurality of teeth formed on it. The teeth have underlying stubs of steel which are integrally formed with and protrude from the cutter. The stubs have flanks which incline toward each other and terminate in a top. A carburized layer is formed on the flanks and the top to a selected depth. The stub has a width across its top from one flank to the other that is less than twice the depth of the carburized layer. A layer of hardfacing is coated on the tops and flanks of the stub, forming an apex for the tooth.
U.S. Pat. No. 6,241,034 discloses a cutter element for a drill bit. The cutter element has a base portion and an extending portion and the extending portion has either a zero draft or a negative draft with respect to the base portion. The non-positive draft allows more of the borehole bottom to be scraped using fewer cutter elements. The cutter elements having non-positive draft can be either tungsten carbide inserts or steel teeth.
Referring now to FIG. 1, which illustrates a milled tooth roller cone rock bit generally designated as 10. The bit 10 consists of bit body 12 threaded at pin end 14 and cutting end generally designated as 16. Each leg 13 supports a rotary cone 18 rotatively retained on a journal, optionally cantilevered from each of the legs (not shown). The milled teeth generally designated as 20 extending from each of the cones 18 may be milled from steel. Each of the chisel crested teeth 20 forms a crest 24, a base 22, two flanks 27, and tooth ends 29.
Hardfacing material may be applied to at least one or each of the teeth 20. In one embodiment, the application of hardfacing is applied only to the cutting side of the tooth as opposed to the other flanks 27 and ends 29 of the teeth 20. In another embodiment, the hardfacing may be applied to all the flanks 27 and ends 29 of the teeth 20.
The rock bit 10 may further include a fluid passage through pin 14 that communicates with a plenum chamber (not shown). In one embodiment, there are one or more nozzles 15 that are secured within body 12. The nozzles direct fluid from plenum chamber (not shown) towards a borehole bottom. In another embodiment, the rock bit 10 has no nozzles 15. In another embodiment, the upper portion of each of the legs may have a lubricant reservoir 19 to supply a lubricant to each of the rotary cones 18 through a lubrication channel (not shown).
Turning now to the prior art of FIGS. 2A and 2B, conventional hardfaced chisel crested teeth generally designated as 40, when they operate in a borehole for a period of time, wear on the corners 44 of the teeth. The prior art tooth consists of a crown or crest 41 having hardfacing material 42 across the crest and down the flanks 43 terminating near the base 45 of the tooth 40.
FIG. 2C shows the prior art tooth of FIG. 2A with a typical axial stress distribution. The prior art teeth (40) typically have a concave axial stress distribution (50) as shown in FIG. 2C.
As heretofore stated the hardfacing material 42 transitioning from the crest 41 towards to the flanks 43 may be very thin at the corners of the conventional teeth 40. Consequently, as the tooth wears, the hardfacing, since it may be very thin, may wear out quickly, and thus expose the underlying steel 47 of the tooth 40. Consequently, erosion voids (not shown) could invade the base metal 47 since it is usually softer than hardfacing material 42.